Generally, transfer devices for vehicles are connected to a transmission and distribute torque from an input shaft to two output shafts eventually to transmit the torque to front and rear wheels. The transfer devices are usually applied with a center differential and an auxiliary transmission mechanism. The center differential automatically adjusts differences in rotational speeds between the front wheels and rear wheels. Some center differentials selectively switch between four-wheel drive mode and two-wheel drive mode. The auxiliary transmission mechanism is usually configured to have two shift stages such as high and low. By combining the shift stages of the auxiliary transmission mechanism with shift stages of a transmission, multiple shift stages are achieved. The center differential and the auxiliary transmission mechanism generally include a gear mechanism such as a planetary gear set. In order to prevent the abrasion and/or overheating of the gear mechanism and supporting portions of an input shaft and an output shaft, the lubrication fluid sealed within a casing of the transfer device for the vehicles is circulated therein to lubricate each portion.
The lubrication systems applied to transfer devices and transmissions are similar in structure. For example, with a lifting system, the lubrication fluid, reserved at the bottom portion of the casing, is lifted by a rotating gear which spreads the lubrication fluid so that the lubrication reaches each of the portions. According to a known lubrication system for an oil seal described in JPH07-071612A, which is an example of the lifting system, the lifted lubrication fluid is received by an oil receiver and flows into a sealing portion by means of a guide rib. In the meantime, because the lubrication fluid is not adequately forced to portions to be lubricated because of the reaction force of the centrifugal force exerted thereon when directly applying the lifted lubrication fluid to the gear which rotates at high speed, the adequate lubrication is unlikely to be provided. Accordingly, a system having a passage in a shaft is also applied as a lubrication system. With the construction of the system having a passage in a shaft, an oil passage in an axial direction and another oil passage in a radial direction are formed inside a shaft, and the lubrication fluid is introduced from an oil receiver to be supplied to desired locations. In case the lubrication fluid is unlikely lifted because of the level difference in a casing and other constructional limitations and/or in case a large degree of flow rate of lubrication fluid is required, a pump which is driven by a rotational shaft may be applied to suck the lubrication fluid.
With known transfer devices which include the center differential and/or the auxiliary transmission mechanism, an oil passage is most likely formed inside an output shaft in order to lubricate gear mechanisms thereof. In case a pump is provided in the transfer device, it is favorable to use an input shaft, which stably rotates without being affected by a gear ratio and a connection and disconnection of the center differential, as a drive source without providing an additional operational power source. Further, in known transfer devices, in order to downsize the transfer devices, an input shaft is shaped approximately in cylindrical form and a pilot bearing is arranged inside of the input shaft to support an output shaft coaxially to the input shaft and to be relatively relatable, so that the length of the transfer device per se is reduced in an axial direction. With this structure, a lubrication fluid supply passage which penetrates through the input shaft is provided for introducing the lubrication fluid from a pump positioned in the vicinity of the input shaft to a passage provided inside the output shaft.
As mentioned above, a transfer device is connected to a transmission. Particularly, for downsizing the transfer device, an input shaft of the transfer device is engaged, by means of a spline, directly to an output shaft of a transmission. Further, with the known structures of the transfer device, the input shaft is required to have the lubrication fluid supply passage and also to support the output shaft. Thus, a spline, the lubrication fluid supply passage, and the pilot bearing are arranged in an axial direction in an internal periphery of the approximately cylindrical input shaft, which makes it difficult to reduce the length in the axial direction and to downsize the transfer device per se.
A need thus exists for a transfer device, which has a shorter length in an axial direction by changing arrangements of a lubrication fluid supply passage.